A shell is defined as a curved structural body with a thickness much less than the radius of curvature of the surface. A shell is essentially characterized by its reference surface, its thickness, and its edges. The reference surface defines the overall shape of the shell and it is a principal factor in the structural behavior of the body.
The usually right angled edges of shells are necessarily thicker or at least stronger than the shell surface itself. In this invention, the shell edges are arcs of great circles lying on a spheroidal surface of revolution. Parabolic, spherical or ellipsoidal surfaces can thus be designed using this inventive concept.
The theory of thin shells has been well developed in the classical mathematical and physical mechanics literature. To a first approximation, the important membrane action of shells implies that their resistance to external loads is carried by internal forces induced within the shell surface, analogous to the skin forces of a balloon in resisting internal pressures.
Although shells are well known in nature and in certain artificial structures, such as airplanes, boats and automobiles, they have not gained wide acceptance because of the difficulties in fabricating and erecting these structures. Most large shells have complicated joint connections which are not easily assembled in the field and the specialized plate and beam members are not easily formed and installed. Spherical surfaces have been inherently more difficult to lay out than rectangular shapes compatible with normally available building materials. Large reflectors have usually not been feasible because of weight, structural complexity, aiming problems and wind loads.
It is the purpose of these specifications to describe a novel but simple method of constructing relatively large shells which can be utilized as solar reflectors or as cost efficient buildings. Large solar reflectors hold considerable promise for concentrating solar energy on a small area, perhaps using a movable focal point which for a sphere is located at half the radius of curvature from the reflecting surface. When used either as a reflector or as a domical building, the shell edges also act as edge arches and thus need only be supported at the corners where the edges join one another.